As a conventional multilayer circuit board it is known that, for example, as shown in FIGS. 1A and 1B, a so-called rigid circuit board 3 which is made by forming a first circuit pattern 2 made of copper foil on a thick insulating substrate 1 made of such as paper impregnated with phenolic resin or paper impregnated with epoxy resin and the like is laminated on and made integral with a so-called flexible circuit board 6 which is made by forming a second circuit pattern 5 made of copper foil on a film 4 made of such as polyimide resin, polyester resin and the like by way of an adhesive insulating layer (for example, epoxy resin adhesive) 7, in which the upper and lower wiring patterns 5 and 2 are connected together by conductive material 8 to form a multilayer circuit board 9. Reference numeral 10 denotes an electric device mounted on the substrate 9, and numeral 11 denotes a solder.
In the multilayer circuit board 9 thus composed, there are such disadvantages that since a distance between the upper and lower patterns 2 and 5 is so large, the connection between both the patterns 2 and 5 through the conductive material 8 is low in reliability; at a connection portion 12 of the electric component 10, both the patterns 2 and 5 can not be directly soldered together by the same solder 11 at the same time for mounting the component thereon; and a fairly small aperture with a diameter less than 1.sub.mm is difficult to be bored through the flexible circuit board 6, which results in that a high quality one or one with high density integration can not be produced. Moreover, in the case of the manufacturing, as shown in FIG. 2, when the wiring pattern 5 of the upper layer is formed by a selective etching method to remove copper foil of uncovered portions on a resist ink 13 formed by printing, the wiring pattern 2 of the lower layer facing a through-hole 14 is eroded or etched (shown by reference numeral 15 in the figure) by etchant. Therefore, it is necessary to cover or fill the through-hole 14 beforehand with resist ink, so the manufacturing process is quite complicated.